The fast-forged process: titanium vs steel
Dr Martin Jackson argues for a new hybrid solid-state process that will support the expansion of titanium alloy use in aircraft.
Titanium alloys have excellent mechanical properties. They have strengths comparable to steel but with half the density, in addition to being resistant to corrosion and biocompatible. However, one of the major drawbacks for titanium alloys is the cost of production, which confines this super-metal largely to components in the aerospace industry, such as jet fighter bulkheads, helicopter rotor hubs, large high-strength landing gear forgings, superplastically formed hollow fan blades and intermediate pressure compressor components, such as blisks.
In such safety-critical components, properties such as low density, high levels of strength and excellent fatigue performance are essential. With large volumes of civil aircraft orders expected over the next decade, increasingly manufactured from carbon composite fuselage and wing structures, there will be a corresponding increase in titanium tonnages for fasteners and high-strength forgings. This is because of titanium’s compatibility with carbon, unlike steel and aluminium alloys, which are more anodic than carbon and can lead to galvanic corrosion.
A step change
With current world mill production capacity at approximately 130,000 tonnes, there will be restricted availability and long lead times for titanium aero-structural components. However, aerospace companies such as Rolls-Royce, Boeing and Airbus have long-term supply agreements with titanium producers, such as PCC TIMET, Arconic and VSMPO-AVISMA. If other sectors such as the automotive industry are to replace steel parts with titanium in high-production vehicles for light-weighting of powertrains and exhaust systems to meet CO2 emissions legislation, there needs to be a step change in the economics of titanium, and additional resources of titanium need to come online over the next decade.
The processing costs of titanium alloy mill product can be broken down into two main areas. The first is ingot production, accounting for around 50% of the total cost, which encompasses ore handling, Kroll process extraction, alloying and melting via double or triple vacuum arc re-melting (VAR) or electron beam cold hearth re-melting (EBCHR). The second costly stage is the multi-step thermomechanical processing in open die hot forging and re-heating, which reduces the grain size and provides the required mill-product shape. The proportion of the cost from downstream secondary processing rises with the complexity of the final component increases as additional costly steps are added, such as closed die forging and high-performance machining. In some extreme cases, such as landing gear components and bulkheads, 80–90% of the forging is machined to swarf.
For titanium to compete with commodity metals, a two-pronged development programme targeting cost reduction in extraction and downstream processing nearer to net shape is required. Over the last few years, with funding from the EPSRC Centre for Doctoral Training in Advanced Metallic Systems, a team of early career researchers at the University of Sheffield, UK, in partnership with UK and EU companies and technology centres in the aerospace, defence and automotive manufacturing sectors, are developing titanium alloys from rutile sand through to the production process.
Rutile sand to titanium alloy powder
Lyndsey Benson, a PhD student within the Sheffield Titanium Alloy Research (STAR) group is working alongside Metalysis Ltd, a South Yorkshire-based company with the global rights to proprietary technology, to exploit a solid state electrolytic process to produce titanium alloys in a particulate form. This is a low-cost and clean electrochemical extraction of metals and synthesis of alloys directly from the mineral precursors, particularly oxides, with the aid of molten salts, originally developed by Fray, Farthing and Chen at the University of Cambridge, UK, and has since scaled up to industry.
Metalysis’ process has many advantages. Unlike the conventional process, high melting point refractory metals, such as tungsten, molybdenum and tantalum can be introduced as mixed oxides with pure TiO2 rutile at the feedstock stage, as opposed to the traditional master alloy addition stage prior to vacuum arc re-melting. More recently, the Metalysis process has been demonstrated to be capable of producing titanium alloy powder/particulate directly from synthetic rutile, which is synthesised via the Becher Process from the iron-rich titanium mineral ilmenite (FeTiO3) – a black sand commonly found in the Earth’s crust in places such as Australia, India and Sri Lanka. Its ilmenite origins means its synthetic rutile feedstock naturally contains a range of remnant elements, principally iron, in addition to small amounts of aluminium and transition metals, such as vanadium and manganese.
Interestingly, very expensive, high-strength titanium alloys employed in aerostructural components, such as landing gear forgings, require small additions of such transition elements to manipulate titanium’s allotropic alpha and beta phases and generate fine-scale microstructures with excellent mechanical properties. Careful control of the upstream Becher Process reduction and associated processes enables different 'flavours' of synthetic rutile to be produced. For example, when applied in a spring, it requires a high volume fraction of beta stabilisers – the chemistry can be controlled at the feedstock stage prior to the Metalysis extraction. This negates the need for more expensive mixed oxide additions as well as master alloys.
The alloy development presents an exciting prospect for the UK as a range of high-strength titanium alloys could produce a one-step solid-state extraction process from specifically tailored synthetic rutile chemistries. This is opposed to the current multi-step method, which consists of the carbo-chlorination of oxides, fractional distillation of chlorides and magnesium reductions before vacuum arc re-melting stages. Today, the Metalysis process arguably remains the most promising low-cost non-melt extraction process to produce alternative sources of titanium alloys from synthetic rutile. The final powder morphology can be either spherical for additive manufacturing processes or angular for solid state processing routes, such as continuous rotary extrusion, FAST-forge and direct powder rolling. Alternatively, lower cost extraction routes such as the Metalysis process are an important part in addressing the cost barriers. However, the downstream processing of such powder feedstock into usable mill product and shapes could reduce costs.
Nick Weston, Research Associate at STAR has been working with the UK’s Defence Science and Technology Laboratory and Kennametal UK to develop an effective hybrid manufacturing processing technology that consolidates a range of titanium particulate feedstocks. This includes Metalysis synthetic rutile-derived alloy powders, commercial powders including the Ti-6Al-4V hydride-dehydride and Ti-6Al-4V machined swarf into net shapes using a two-step solid-state processing technology, as opposed to the conventional multi-stage forging and heating route. The FAST-forge downstream consolidation route, exploits field assisted sintering technology (FAST) or spark plasma sintering with traditional precision hot forging processes. Rapid sintering can be achieved for near net shapes without the need for process limiting steel canning and vacuum degassing and welding steps. Shaped graphite dies are used to provide a range of pre-forge billet geometries. A better structural integrity can be generated through the FAST-forge process compared with additive manufacturing techniques, because of the exploitation of precision hot forging to refine the microstructure and provide (grain) flow lines into forged parts. For safety-critical components, thermomechanical processing is essential to refine the microstructure to achieve good fatigue, toughness and strength properties.
The FAST-forge technology could aid engineers with more flexibility, enabling them to impart controlled levels of deformation and graded alloy chemistry in defined regions to generate specific mechanical properties of the component. In addition, the hybrid process has benefits for material use and reuse, leading to improved buy-to-fly ratios needed for the aerospace industry when manufacturing titanium alloy components. The two exciting developments described will be combined over the next several years. SAFRAN Landing Systems, UK, will lead developments to accelerate the FAST-forge
technology of Metalysis titanium alloy powder into small-to-medium landing gear components, in collaboration with the University of Sheffield, Metalysis and the Advanced Forming Research Centre at the University of Strathclyde, UK. By 2018, the consortium aims to demonstrate the manufacture of safety-critical titanium alloy components from rutile sand in a total of three solid-state processing steps. Such technology will be a great addition to the UK’s world-leading high-value manufacturing capabilities and new source of low-cost titanium components for a range of sectors.
Today, titanium is confined to niche applications in high-performance vehicles. But it is the automotive sector that could be the greatest benefactor of the low-cost processing developments, particularly in powertrain and exhaust systems. For many years, it has been demonstrated that titanium connecting rods, turbochargers and valves could benefit the industry in terms of increased performance, mass reduction and lower emissions, but the high cost makes it very unattractive. However, within the next five-to-ten years, FAST-forged profiled shapes could be used not only for emerging low-cost titanium alloys, such as Metalysis synthetic rutile derived products, but also in the reuse of machined swarf in automotive parts such as connecting rods or rocker arms. Bringing together machining, testing capabilities and expertise in qualifying aerospace grade materials will ensure that the titanium components, which will be manufactured under this new process, become a benchmark for the industry.
Dr Martin Jackson is Director of Aerospace Engineering at the University of Sheffield, Director of Sheffield Titanium Alloy Research group, UK, and the IOM3 representative on the World Titanium Conference Organisation Committee. With thanks to Dr Ian Mellor (Metalysis), Dr Adam Tudball (Kennametal UK) and Dr Matthew Lunt (DSTL).